1. Field of the Invention
The present invention relates to mechanisms for extracting water from a web of-material, and more particularly from a cellulosic fibrous web being processed into a paper product on a papermaking machine. Specifically, the present invention is an impermeable belt comprising a polymeric-resin-coated and impregnated base support structure which may include polymeric-resin-coated elements. The belt is designed for use on a long nip press on a papermaking machine. The belt may also be used in other papermaking and paper-processing applications, such as calendering and sheet transfer.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed on a forming fabric by depositing a cellulosic fibrous slurry thereon. A large amount of water is drained from the slurry during this process, after which the newly formed web proceeds to a press section. The press section includes a series of press nips, in which the cellulosic fibrous web is subjected to compressive forces designed to remove water therefrom. The web finally proceeds to a drying section which includes heated dryer drums around which the web is directed. The heated dryer drums reduce the water content of the web to a desirable level through evaporation, thereby completing the manufacture of a paper product from the web.
Rising energy costs have made it increasingly desirable to remove as much water as possible from the cellulosic fibrous web prior to its entering the dryer section. The dryer drums are often heated from within by steam and related costs can be substantial, especially when a large amount of water needs to be removed from the cellulosic fibrous web.
Traditionally, press sections have included a series of nips formed by pairs of adjacent cylindrical press rolls. In recent years, the use of long nip presses has been found to be advantageous over the use of nips formed by pairs of adjacent press rolls. This is because the longer the time a cellulosic fibrous web can be subjected to pressure in the nip, the more water can be removed there, and, consequently, the less water will remain behind in the web for removal through evaporation in the dryer section.
The present invention relates to long nip presses of the shoe type. In this variety of long nip press, the nip is formed between a cylindrical press roll and an arcuate pressure shoe. The latter has a cylindrically concave surface having a radius of curvature close to that of the cylindrical press roll. When the roll and shoe are brought into close physical proximity to one another, a nip, which can be five to ten times longer in the machine direction than one formed between two press rolls, is formed. This increases the so-called dwell time of the cellulosic fibrous web in the long nip while maintaining an adequate level of pressure per square inch of pressing force. The result of this new long nip technology has been a dramatic increase in dewatering of the cellulosic fibrous web in the long nip when compared to conventional press nips on paper machines.
A long nip press of the shoe type requires a special belt, such as that shown in commonly assigned U.S. Pat. No. 5,238,537 to Dutt. This belt is designed to protect the press fabric supporting, carrying and dewatering the cellulosic fibrous web from the accelerated wear that would result from direct, sliding contact over the stationary pressure shoe. Such a belt must be provided with a smooth, impervious surface that rides, or slides, over the stationary shoe on a lubricating film of oil. The belt moves through the nip at roughly the same speed as the press fabric, thereby subjecting the press fabric to minimal amounts of rubbing against the surface of the belt.
Belts of the variety shown in U.S. Pat. No. 5,238,537 are made by impregnating a woven base fabric, which takes the form of an endless loop, with a synthetic polymeric resin. Preferably, the resin forms a coating of some predetermined thickness at least on the inner surface of the belt, so that the yarns from which the base fabric is woven may be protected from direct contact with the arcuate pressure shoe component of the long nip press. It is specifically this coating which must have a smooth, impervious surface to slide readily over the lubricated shoe and to prevent any of the lubricating oil from penetrating the structure of the belt to contaminate the press fabric, or fabrics, and cellulosic fibrous web.
The base fabric of the belt shown in U.S. Pat. No. 5,238,537 may be woven from monofilament yarns in a single- or multi-layer weave, and is woven so as to be sufficiently open to allow the impregnating material to totally impregnate the weave. This eliminates the possibility of any voids forming in the final belt. Such voids may allow the lubrication used between the belt and shoe to pass through the belt and contaminate the press fabric or fabrics and cellulosic fibrous web.
When the impregnating material is cured to a solid condition, it is primarily bound to the base fabric by a mechanical interlock, wherein the cured impregnating material surrounds the yarns of the base fabric.
While the belts shown in U.S. Pat. No. 5,238,537 have proved to be durable, reliable and long-lived on long nip presses, improvements both in the structure of such belts and in methods for their manufacture are continually being made. Some of the improvements are driven by the need to prevent the polymeric resin coating from delaminating from the base fabric and relate to means for improving the mechanical interlock between the base fabric and the coating. Other improvements relate to the structure of the base fabrics themselves, and are designed to make the base fabrics stronger, more durable, or to the exact dimensional specifications required for a given application. Still other improvements relate to the coating processes, and have as their object the complete impregnation of the base fabric and the provision of a uniformly thick coating of polymeric resin material on the inner surface of its endless configuration without the step of inverting (turning inside out) the belt during the manufacturing process.
The present invention relates, in part, to the need to improve the interlock between the base fabric and the polymeric resin coating of a long nip press belt. More specifically, the present invention is a long nip press belt having a base support structure which may include polymeric-resin-coated yarns, wherein the polymeric resin material used to coat the yarns has an affinity for the polymeric resin material used to coat the belt as a whole, so that the coating on the belt forms a chemical, as well as a mechanical, interlock with the base fabric. In addition to being useful as a long nip press belt, the present invention may also be used in other papermaking and paper-processing applications, such as calendering and sheet transfer.
Accordingly, the present invention is a resin-impregnated endless belt for a long nip press. The belt may also be used on a calender of the shoe type, as both a long nip press and a calender of that type comprise a cylindrical press roll and an arcuate pressure shoe which together define a nip therebetween. The resin-impregnated endless belt passes through the nip in direct sliding contact with the arcuate pressure shoe, and separates a fibrous web, and perhaps a press fabric or fabrics supporting the fibrous web, from the arcuate pressure shoe, thereby protecting the fibrous web, and the press fabric or fabrics, from damage by direct sliding contact with the arcuate pressure shoe and from contamination by any lubricant on the arcuate pressure shoe. The belt may also be used for other papermaking and paper-processing applications, such as on a roll calender or as a sheet transfer belt.
The resin-impregnated endless belt comprises a base support structure which may include, among the plurality of elements from which it is formed, a plurality of coated elements having a coating of a first polymeric resin material. The base support structure is in the form of an endless loop having an inner surface, an outer surface, a longitudinal direction and a transverse direction.
The base support structure may take one of several forms. In one embodiment, the base support structure may be a woven structure, such as that shown in U.S. Pat. No. 5,238,537, the teachings of which are incorporated herein by reference. Such a woven structure is woven from a plurality of warp yarns and a plurality of weft yarns, wherein the plurality of coated elements having the coating of the first polymeric resin material is included among at least one of the pluralities of warp yarns and weft yarns.
In a second embodiment, the base support structure comprises a structure braided from a plurality of yarns, wherein the plurality of coated elements having the coating of the first polymeric resin material is included among the plurality of yarns. For example, the braided structure may have a plurality of braided yarns, wherein, in each of the layers, at least one yarn thereof extends into a contiguous layer to form an interlock therebetween. The layers are therefore interlocked with one another, and are unable to delaminate from one another. Such a braided structure is shown and described in commonly assigned U.S. Pat. No. 5,501,133 to Brookstein et al., the teachings of which are incorporated herein by reference, and a resin-impregnated endless belt having such a braided structure for a base fabric is shown and described in commonly assigned U.S. Pat. No. 5,772,848 to Dutt, the teachings of which are also incorporated herein by reference.
In a third embodiment, the base support structure comprises a spirally wound fabric strip, wherein the fabric strip has a width smaller than a width of the base support structure. The base support structure has a plurality of non-overlapping, abutting turns of the fabric strip joined along a continuous spiral seam, as shown and described in commonly assigned U.S. Pat. No. 5,360,656 to Rexfelt et al., the teachings of which are incorporated herein by reference. The fabric strip may be woven from a plurality of warp yarns and a plurality of weft yarns, wherein the plurality of coated elements having the coating of the first polymeric resin material is included among at least one of the pluralities of warp yarns and weft yarns. Alternatively, the fabric strip may be a non-woven mesh fabric, such as that shown in U.S. Pat. No. 4,427,734, or a fabric having open spaces between its component yarns, including knitting, lace, net, crochet, braiding and the like. Some portions of the non-woven mesh fabric, or some yarns of the knitted and braided fabrics, are coated with the first polymeric resin material.
In yet other embodiments, the base fabric may comprise a layer of yarns oriented in the longitudinal direction and a layer of yarns oriented in the transverse direction, the yarns in the two layers not being interwoven with one another. A belt having a base fabric of this type is shown and described in U.S. Pat. No. 5,118,391 to Matuschczyk et al., the teachings of which are incorporated herein by reference. The base fabric may also be a spiral-link belt of the variety shown in many U.S. patents, such as U.S. Pat. No. 4,567,077 to Gauthier, the teachings of which are also incorporated herein by reference.
Finally, the base support structure comprises a laminated structure having a plurality of layers, each of the layers being one of the six above-described structures and each including a plurality of coated elements having a coating of a first polymeric resin material having a melting point lower than the material of the yarns themselves. This enables the plurality of layers to be laminated together using heat and pressure. The layers may also be laminated together by driving a fiber batt therethrough by needling or hydroentangling. In such a case, at least some of the fibers in the batt may be of the first polymeric resin material, or may be coated with the first polymeric resin material. Where the base support structure is a laminated structure, one of the layers may be an on-machine-seamable fabric.
In any event, at least one of the inner and outer surfaces of the base support structure has a coating of a second polymeric resin material. The coating impregnates the base support structure and renders it impermeable to liquids, such as oil and water, and is ground and buffed to provide it with a smooth surface, and the belt with a uniform thickness. The second polymeric resin material has an affinity for the first polymeric resin material coating the plurality of coated elements. As a result, the coating of the second polymeric resin material on the base support structure as a whole establishes a chemical, as well as a mechanical, interlock with the plurality of coated elements, having the coating of first polymeric resin material, of the base support structure, and/or with batt fibers, made of or coated with the first polymeric resin, which may be attached to the base support structure.
The present invention will now be described in more complete detail with frequent reference being made to the figures, which are listed and identified below.